The subject of the invention is an improvement in a catalytic reforming process. Catalytic reforming is a refining process that uses selected operating conditions and selected catalyst to convert naphthenes and paraffins to aromatics and isoparaffins. Hydrocarbon molecules are predominantly rearranged without altering the number of carbon atoms in the molecule.
Petroleum is subjected to fractional distillation in fractional distillation towers including a pipe still, a vacuum pipe still and associated distillation towers. The resulting fractions range from the lightest hydrocarbon vapors including methane, ethane, ethylene, propane, and propylene to a heavy vacuum residuum having an initial boiling point of 1100.degree. F. Intermediate between propane/propylene and the heavy vacuum residuum fractions are a number of intermediate fractions. The cut points of each of those intermediate fractions is determined by refinery configuration and product demand. These intermediate fractions include naphtha, kerosene, diesel. gas oil and vacuum gas oil. Any of these fractions which is taken directly from the fractional distillation of crude petroleum is referred to as "straight run."
A large body of technology has been developed for the conversion of one intermediate fraction to another. Converted fractions are by definition not straight run and are referred to as coker or cracked. Converted fractions differ from straight run fractions, particularly in the distribution of suibstituent components in the fraction. The substituent components include paraffins, naphthenes and aromatics.
In the catalytic reforming process, a hydrocarbon fraction containing paraffins and naphthenes is contacted with a catalyst which promotes the dehydrogenation of naphthenes to aromatics; isomerization of paraffins and naphthenes; hydrocraeking of naphthenes and paraffins and other reactions to produce an octane enhanced liquid and hydrogen.
Catalysts effective in carrying on these reactions are referred to as dual function catalysts because they exhibit the capability of both selectively cracking and hydrogenating. Dual function catalysts often demonstrate high initial activity. However, these catalysts are particularly susceptible to decline in activity in part due to deposition of coke on the catalyst. When the activity declines below a certain level, which depends on the product desired and the plant capabilities then the catalyst must be regenerated. This often results in substantial down time. Catalyst activity over time, referred to as stability, is therefore more critical in evaluating catalyst performance for commercial use than high initial activity.
It is known to increase catalyst and absorber specificity by setting the pore size to some predetermined range. For instance, to preferentially absorb low branched hydrocarbons from raffinate, U.S. Pat. No. 5,135,639 states the pore size is the key criterion and that suitable pores had diameters between 4 and 6 Angstroms. Because the feed in this process is heavier cracked naphtha such as Coker Naphtha. and the molecules in this material are small as a result of the cracking, it was long thought that small pores, and the resultant greater surface area. were preferred reforming catalysts. For instance, U.S. Pat. No. 5,437,783 describes a reforming catalyst with a pore volume of between 0.3 cubic centimeters (cc) per gram to 0.6 cc per gram, with the further stipulation that 85% of the pore volume be composed of pores with diameters smaller than 100 Angstroms. U.S. Pat. No. 4.969,990 describes a hydroprocessing catalyst with a narrow pore size distribution with the mode diameter between 70 and 90 Angstroms.
Activity is a measure of the ability of a catalyst to convert reactants to products at specified reaction conditions. Specified reaction conditions are referred to as severity and include: temperature, pressure, residence time and hydrogen partial pressure. Activity is reported as the research octane number (RON) of the debutanized liquid (DBL) product from a given feedstock. An alternate method of measuring activity is the temperature required to achieve a specified octane number, e.g., temperature to produce a 101 RON DBL product.
Stability refers to the rate of chance in activity for a given feedstock. Typically, activity decline is measured as the rate of increase in reactor inlet temperature to maintain a specified octane number for the DBL product. A lower rate of temperature increase per unit time is a better stability because it provides a longer run length until end of run (maximum) temperature is reached. Pilot plant stability data are, due to severity of conditions, the feedstocks, the reactor design, or a combination thereof, often 2 orders of magnitude worse than commercial plant stability. When pilot plant stability is mentioned in the claims, these values are in relation to the reaction conditions and feedstocks similar to those used in the examples.
Selectivity or yield is the relative amount of the desired DBL product produced from a feedstock. Yield stability is the rate of decrease in DBL product produced from a feedstock per unit of time.
Factors which adversely influence catalyst activity, and therefore stability, include covering of active surface area by coke and by the deposition of poisons such as sulfur and metals onto the active catalyst.
What is needed in the industry is reformer catalyst formulations that have good selectivity, good activity, and excellent stability.